Noninvasive diagnosis and measurement of blood glucose concentration has attracted tremendous attention in the past two decades because of the emergence of diabetes as a public health epidemic. In current practice, all available portable end-user devices for measuring blood glucose require puncturing of the fingertip to obtain a blood sample. The blood sample is then placed on a test strip that is read by a meter to indicate the glucose concentration. These devices are very compact and reasonably accurate, but puncturing the fingertip to obtain a blood sample is inconvenient, painful, and poses a risk of infection. For users who are elderly or vision-impaired, these smaller devices may also be difficult to use. In addition, the cost of systematic and continuous use of single-use test strips is high. Noninvasive devices for accurately measuring blood glucose may address the above listed limitations of invasive glucose measurement devices, but are not commercially available at present.
Noninvasive measurement of glucose offers the potential for increased frequency of testing and more responsive control of blood glucose concentrations through concomitant adjustment of insulin doses. Noninvasive detection techniques also offer the potential for accurate, portable, closed-loop systems for monitoring and regulating insulin dosage without an indwelling sensor. These prospective advantages have led to considerable interest in the commercialization of noninvasive glucose monitoring devices.
One of the possible methods of performing noninvasive glucose measurements includes measuring tissue attenuation of light radiation in the near infrared energy spectrum (approximately 650 nm to 2700 nm). U.S. Pat. No. 5,099,123 to Harjunmaa et al. (the '123 patent), which is incorporated herein in its entirety by reference, discloses a balanced differential (or OPTICAL BRIDGE™) method for measurement of a target analyte concentration (e.g. glucose concentration) in a sample background matrix (e.g. body fluids and tissue) of a sample (e.g. an earlobe). To obtain measurement data, the method utilizes two wavelengths: a principal wavelength, which is absorbed by the target analyte, and a reference wavelength, which is less absorbed by the target analyte, that is before the measurement selected (i.e. fine tuned from its nominal or initial value) by the OPTICAL BRIDGE™ balancing process. The OPTICAL BRIDGE™ balancing process includes adjusting the reference wavelength so that the sample background is differentially (between the principal and reference wavelengths) “invisible”. By making the sample background differentially invisible, any difference between the principal wavelength and the reference wavelength measurement data can be attributed to an amount of target analyte in the sample.
Subsequently, in U.S. Pat. No. 5,178,142, which is incorporated herein by reference, Harjunmaa et al. disclosed a method of changing the extracellular to intracellular fluid ratio of the tissue matrix by varying the mechanical pressure on the tissue, and performing the OPTICAL BRIDGE™ balancing when there is a minimum level of analyte present in the sample.
In U.S. Pat. No. 7,003,337, which is incorporated herein by reference, Harjunmaa et al. disclosed continuous estimation of the volume of sample fluid (e.g. blood) containing the target analyte (e.g. glucose) within the sample (e.g., earlobe) using another radiation (e.g. green light which is absorbed by hemoglobin). The fluid volume estimation is combined with the measurement data corresponding to the principal and reference wavelengths to provide for an improved calculation of the analyte concentration. Further, in U.S. Pat. No. 8,175,666 (the '666 patent), which is also incorporated herein by reference, Harjunmaa et al. disclosed a method of producing a radiation beam using three fixed-wavelength diode lasers, wherein varying the relative intensities of the two fixed-wavelength reference diode lasers produces an effect comparable to tuning (i.e. varying) of the reference wavelength.
Other related patents include U.S. Pat. Nos. 5,112,124; 5,137,023; 5,183,042; 5,277,181 and 5,372,135, each of which is incorporated by reference herein in its entirety. Related patent applications include U.S. patent application Ser. Nos. 13/835,143 and 13/441,467, each of which is incorporated by reference herein in its entirety.